Do magnetic fields slow accretion onto AGN stars?

Jun 16, 2022 09:20 · 374 words · 2 minute read astronomy science research

I sometimes have research ideas that I think are cool, but that don’t make sense for me to pursue. I generally just make a note of them and move on. This is the nineteenth post in a series describing some of the ideas I’ve accumulated. This idea was suggested to me by Yuri Levin and I’ve just fleshed it out a little below.

Do magnetic fields slow accretion onto AGN stars?

What’s the idea?

Stellar evolution in AGN disks is expected to produce massive stars that exist in a balance between wind mass loss and accretion. Calculations of this balance so far neglect magnetic fields in determining the accretion rate. AGN disks almost certainly have significant magnetic fields, though, and these just get amplified in the accretion stream. The resulting magnetic pressure could well slow the stream and make AGN stars less massive.

Why is this important?

Stars in AGN disks potentially contribute to a wide range of disk phenomena, so understanding their evolution is crucial for understanding the disks in which they live. It is also intrinsically interesting, I think, to understand the lives of stars in such extreme environments.

How can I get started?

In full this problem probably requires 3D radiation magnetohydrodynamics simulations, but there is probably still something to be done analytically in the style of the original Bondi accretion calculations. Here’s a rough take:

When flux is conserved, $B \propto r^{-2}$, so $P_{\rm mag} \propto B^2 \propto r^{-4}$. This increases faster with falling $r$ than any other source of pressure support in the accretion stream, and so could well slow the stream.

In gas-dominated regime, accretion halts when $B^2 \sim \rho v^2 \sim P_{\rm agn, gas} (r/r_B)^{-5/2}$ (that last step comes from here). At the Bondi radius $r_B$, $B^2 \sim P_{\rm agn}$, so halts around $r_B$ up to factor of order unity. The accretion rate is then set by the effective magnetic diffusion, which sets the rate at which the magnetic pressure in the stream can decrease.

So the next step is to think about what processes can extract magnetic energy from the accretion stream and return it to the AGN disk. That’s probably some turbulent diffusivity, which can probably be estimated self-consistently with the velocity in the stream…

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